What determines the rate of fire of a machine gun, and how can that rate of fire be determined or changed from a design perspective? Let’s talk about pressure, mass, and distance…
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How it did affected Rate-of-Fire of said design?
There are lots of things that change the rate of fire on a machine gun depending on the mechanical operations
One for example on the roller lock
Bolt gap can change operational speeds The locking angle can either release the system quicker or keep it longer in battery this controls the gas that is used in the system the other thing that can change the rate of fire is the recoil Spring a stronger Recoil Spring will cause the weapon to function at a higher rate of speed with a higher rate of fire, also ammunition plays a big role in these systems Due to the gas pressure a stronger and more powerful ammunition will make the weapons rate of fire increase due to overpressure. Recoiling buffing Can also play a role in the rate of Fire in these Machine guns I Am Only speaking on the roller lock system.
Also I forgot to mention resistance In the feed mechanism can play into Rate of fire.
For example an HK21 if the belt is heavy you will see the rate of fire change when it gets to the end of the belt it will speed up This is cause because of resistance of the heavy belt at the beginning of the cycling rate and as the belt empties the resistance is lightened speeding the weapon back up to its normal rate of fire.
The easiest way to think about this is in terms of energy: How much are you tapping out of the cartridge, and what are you using to run the mechanism?
As well, what are your energy returns coming out of the recoil spring system/buffer?
Work those out, understand them, put them into an equation and you can work out what your theoretical rate of fire ought to be. Once you’ve done that, and validated it by actually working with the mechanism, then you can begin to understand what all goes into the rate of fire and possibly be able to work out how to adjust it.
You’ve got an energy budget to work with: How much can you take out of the cartridge energy before affecting ballistic performance? How much can you put into the recoil assembly, and how efficiently do you get that back?
It is possible to change rate of fire through very simple and entirely counter-intuitive means. M60 machineguns, for example? You can, when the damn things are worn in, use the sear to reduce the rate of fire by riding it juuuuuust enough to get the sear to barely catch the sear notch on the op rod. You want full-rate? Pull the trigger and keep it down.
You can’t do that on other designs, for whatever that’s worth.
It’s all about the energy; lots of energy, low-friction on the recoil/feed/locking systems? You’re producing a high rate of fire. Change any of those, and you’re lower. Or, even higher…
What’s interesting is to observe a change in the rate of fire from a difference in mag spring strength. If you were to take a very old, very worn M16 magazine, put it into an M249 in magazine-fed mode, and you’d get one rate of fire. Do that with a brand-new HK steelie? One with a heavy spring? Different rate of fire, notable enough that you could hear it. The HK mags were usually slower, which I ascertain stemmed from the system having to do more work to get the rounds out.
I don’t think people really appreciate how carefully balanced fully-auto systems are, or what huge effects seemingly minor changes can produce in performance.
Awesome video Ian! I am very interested in devices that reduce the rate of fire in a machinegun. I think this would make an excellent video.
Then this https://guns.fandom.com/wiki/McCrudden_light_machine_rifle might be of interest for you.